Natural gas hydrate (NGH) is physically unique among gas resources that occur in the deepwater environment of deep continental shelf areas and continental slopes. The physical nature and occurrence of NGH, the potential volumetric scale of the resource, well understood petroleum system, and other key parameters allow for exploration and production opportunities that do not apply to conventional deepwater gas deposits. These allow for relatively inexpensive adaptation of existing technologies in a less robust form and the development of new technologies and practices to be applied to NGH that will have a strong bearing on their commerciality.
NGH is a non-stoichiometric solid, crystalline material composed of water molecules forming cage structures that are predominantly occupied by hydrocarbon gas molecules. Methane is the dominant gas found in naturally occurring NGH, although higher density hydrocarbons such as ethane, propane, and butane, and non-hydrocarbon gas molecules, for instance such hydrogen sulfide, nitrogen, and carbon dioxide may be found as traces, can also occur in compound NGH. Herein, all types of natural gas hydrate are referred to as (NGH) to include all species for the sake of simplicity.
NGH forms spontaneously under the right combinations of pressure and temperature conditions when there is sufficient natural gas flux within a zone of NGH stability that extends downward from the seafloor to some depth determined by rising temperature. Concentrations of NGH occur in water depths of about 500 m and greater in the open ocean and shallower in Polar Regions. For example, the Nankai deposit, which lies about 200 m below the seabed at a water depth of about 1 km near the edge of the continental shelf SE of Tokyo Bay, Japan is currently the best example of a potentially commercializable NGH deposit. Large concentrations are controlled by geology and a large number of the concentrations may be large enough to constitute an economic natural gas resource. Because the projected amount of NGH in reservoir situations is very large, on the order of 43,000 trillion cubic feet of gas, even a relatively small fraction of the current estimate of gas-in-place means that NGH could constitute the largest recoverable natural gas resource on Earth.
Depressurization has proven as the NGH conversion technique and which was used during the first and successful Japanese technical production test near the edge of the continental shelf SE of Tokyo Bay, Japan. Use of this conversion technique enables a set of opportunities and new risks that drive technology development, particularly in the fields of drilling and production, exist because pressures in the NGH reservoir will be much lower than in conventional deposits and the geotechnical confinement will be much less physically secure. In addition, the strength of the reservoir will decrease with production of the NGH to a much greater extent than a conventional gas deposit as solid NGH is replaced by gas and water.
Oceanic NGH reservoirs are very unlike deepwater conventional reservoirs although they both occupy sediment porosity and displace water. Conventional gas traps are deeply buried and mechanically strong. NGH reservoirs occur in only partially consolidated marine sediment within no more than about 1 km from the seafloor. Conventional gas exists in its reservoir and flows to the wellhead at high pressures and temperatures. NGH deposits, in contrast, consist of solid crystalline NGH that when is converted to its constituent gas and water by depressurization, results in pressure in the producing well system that are lower than reservoir formation pressure and temperatures that are no higher than a few tens of degrees C.
Typical well casing of conventional vertical wells consists of a series of pipe sections joined together using screw, bayonet, or other connections. Casing sections are inserted down into the wellbore from or through the seafloor. A casing well string consists of a series of constant diameter sections that decreases in diameter with depth in a series of stages. This is because only a certain length of casing stage can be mechanically inserted into the well before drag from the bounding rocks or sediments effectively limit the depth of each section. Depths of sections vary based on the geological materials, the oversize of the well bore, and drag that can be caused by a number of factors such as, but not limited to, straightness of the well, the yielding character of the bounding materials, and insertion force available. Because the external surface of casing is not everywhere tight against the rock walls, cementing at intervals is used to fill the annular space between the casing and the rock wall. Cementing is meant to provide a very solid anchor to apparatus at the wellhead, such as the blowout preventer, and within the well to provide a tight seal between the casing and the rock that prevents movements of gas and liquids along the outer surface of the casing.
Production from the reservoir begins with the transfer of gas and unwanted materials such as water and solid particulates through permeable perforated intervals in casing within a reservoir pay zone. Flow control often involves the use of sand screens and gravel packs, which diminish the amount of sediment grains by slowing the flow, which reduces the carrying potential of gas and fluids migrating from the reservoir into the well.
While conventional well casing systems and methods are suitable for conventional gas deposits, they are not suitable for NGH deposits. Accordingly, there is a need for new well casing systems and methods that are suitable for use in the recovery of hydrocarbon gas from deepwater NGH deposits.